Once Nixon knew he was doomed, he dumped Spiro Agnew and brought Ford in the be Vice President. The reason was simple. Among the items sought after in the break-in of the Democratic National Headquarters were photos taken in Dealey Plaza, Dallas, Texas on November 22nd, 1963 that showed three of the "tramps" clearly enough to identify two of them as E. Howard Hunt (the reason he led the break-in) and Frank Sturgis, both Nixon henchmen in 1963.
(The photos eventually did resurface in an expose published by the Liberty Lobby. Hunt sued and lost when witnesses were found that placed him in Dealey Plaza at the time.)

Remember that Nixon was a shoe-in for re-election against McGovern because of all the dirty tricks done by Segretti and his ilk at CREEP. Nobody could understand why the break-in had happened at all. A full investigation of the motive for the Watergate break-in would have unraveled the cover-up of the JFK assassination, which is why Nixon resigned rather than allow a Senate trial.

Gerald Ford had been on the Warren Commission that covered up the conspiracy and put the blame for the killing of JFK on the lone gunman, Oswald. So, prior to resigning, Nixon tossed Agnew and brought in Ford knowing that Ford, likely to be as threatened by exposure of the cover-up as anyone else, would work to protect the secret and by extension, Nixon.

Yes, there was probably an explicit understanding that Ford would pardon Nixon as the price for being made VP, then President. But Ford's pardon protected Ford as much as Nixon.